Gas separation has been the subject of considerable research for many years. For example, NASA Kennedy Space Center advanced the state of the practice of gas separation to capture and remove carbon dioxide (CO2) from the International Space Station and other space habitats. Gas separation technology also has the potential to greatly reduce the amount of carbon dioxide released into earth's atmosphere by capturing and removing CO2 from combustion processes such as power plant operations. Furthermore, CO2 captured during gas separation has many useful applications, including crude oil recovery, methane and biomass production, and bioregenerative applications.
Polymeric membrane is a commonly employed gas separation technology that is characterized by positioning a selective barrier made of a polymeric material between two fluid phases. Polymeric membranes have enjoyed some commercial successes, even though their selectivity for CO2 over other gases is very low. By way of definition, selectivity is a measure of gas separation efficiency expressed as the ratio of the number of two different molecules permeating a membrane per unit time. For example, if 6000 CO2 molecules and only 1 N2 permeated the membrane per minute, the membrane selectivity would be 6000:1 with respect to these molecules. Over the last decade, the CO2 reduction efficiency demonstrated by 80 large-scale polymeric membrane units installed worldwide has averaged only 25 percent of the initial quantity present in the feed. These large scale commercial systems use polymeric membranes including polyamides to capture or reduce CO2.
Immobilized liquid membrane (ILM) is a type of facilitated transport membrane (FTM) that features a liquid solution immobilized in the pores of a polymeric or ceramic substrate by physical forces. The liquid solution typically consists of a carrier and a solvent. The carrier reacts reversibly with certain gas species such that gas molecules of interest are allowed to permeate the carrier while other gas molecules are not allowed to permeate the carrier.
Compared to conventional polymeric membranes, ILMs have the potential to provide higher gas fluxes and improved selectivities for reacting gas species such as carbon dioxide and olefins, particularly at low concentrations in gas separation. As a point of reference, U.S. Pat. No. 3,396,510 to Ward discloses achievement of selectivity ratios for CO2/O2 of 1100:1. During the subsequent decades, work on immobilized membranes has continued with measured CO2 permeability over N2 and CO in the laboratory in excess of 6000:1.
Despite the advantages offered by the immobilized liquid membranes, commercialization of these membranes has been hampered by the inherent limitation of stability of the liquid membranes. For example, the volatility of conventional immobilized liquids results in their tendency to evaporate when confronted by long-term exposure to the vacuum of space. Consequently, immobilized liquids generally cannot be utilized as an interface between, for example, an extravehicular mobility unit (EMU) and space. Similarly, applications such as removal of carbon dioxide from the flue gas of coal-fired power plants tend to volatilize immobilized liquids. The main design challenges contributing to the instability of ILMs lack of chemical bonding of the liquid to the supporting membrane, evaporation of the liquid, and the limited differential pressure the membranes could tolerate.
Typically, immobilized liquid membranes that rely only on support pores for strength suffer from two major problems: (1) the liquid evaporates during prolonged exposure to gaseous mixtures and (2) the liquid is only trapped in the matrix of the supporting media and can only withstand very small differential pressures. Unfortunately, factors that allow higher differential pressures (specifically, smaller pore diameter and longer path) also tend to decrease the selected gas permeation rate. Because of these problems, immobilized liquid membranes have found limited use and commercial availability. However, the gas separation industry is experiencing advancements in designs to stabilize immobilized liquid membranes, some of which may be pertinent to certain aspects of manufacturing and implementing ILM-based gas separation solutions.
U.S. Pat. No. 6,958,085 to Parrish discloses a design to overcome the membrane stability problem, which includes microencapsulation of the immobilized liquid in a gas permeable polymer, followed by dispersion of the microcapsules on the surface of a gas permeable support membrane. However, although the membrane proved to be physically strong, the initial systems could not overcome problems associated with leakage around the microcapsules.
ILM stabilization and liquid leakage are both impacted by the type of polymeric wall material used to contain the immobilized liquids. In a study by Scholes, CO2 separations from N2, O2 and H2O using polymeric membranes which were generally nonporous and which follow the solution-diffusion permeation mechanism were reviewed (See Scholes, C. A., Kentish, S. E., and Stevens, G. W., Carbon Dioxide Separation through Polymeric Membrane System for Flue Gas Application, Recent Patents on Chemical Engineering, 2008, 1, 52-66). The solution-diffusion mechanism was based on the solubility of the specific gas and its diffusion through the dense membrane matrix. In this case, the physical-chemical interaction of a gas with the polymer matrix determines its concentration in the membrane, which is directly proportional to the permeability coefficient. The other important factor is the diffusion coefficient which describes the mobility of the gas in the polymer membrane. If follows that, for microencapsulation of the immobilized liquid, the type of microcapsule wall material and its thickness are critical factors. The permeability of gases through the immobilized liquid is primarily solubility dependent, which is a fact that has been established by earlier immobilized liquid membrane development.
Scholes reported that much work has been done in an attempt to produce efficient membranes to remove carbon dioxide from natural gas for the utility industries, leading to commercial uses of polymeric membranes that include cellulose acetate, polyimides, polyamides, polysulfones, polycarbonates, and polyetherimides. Dortmundt also reported that the commercially viable membranes for CO2 removal are polymer based, and his examples were the same as those reported by Scholes (See Dortmundt, D. and Doshi, K., Recent Developments in CO2 Removal Membrane Technology, UOP 1999, Lecture Presented Mar. 19, 2004). Dortmundt also confirmed that Fick's Law, shown as follows, is widely used to approximate the solution-diffusion permeation process:
  J  =            k      ×      D      ×      Δρ        l  where:
J is the membrane flux of CO2 through the membrane per unit area
K is the solubility of CO2 in the membrane
D is the diffusion coefficient of CO2 through the membrane
Δp is the partial pressure difference of CO2 between the feed (high pressure) and permeate (low pressure) sides of the membrane
I is the membrane thickness
Note: The diffusion coefficient and solubility are often combined and called permeability (P).
Currently employed polymeric membranes require higher pressures to achieve separation of gases. For example, Scholes reported pressures in the range of 60 psi (110 kPa) were used to force CO2 through the polyvinyl acetate membranes with a permeability of 3.1, while cellulose acetate wet with 2N KHCO3 and 0.5 N NaAsO2 had a permeability of 2000 barrier at a pressure of 0.5 psi (4 kPa). The cellulose acetate wet with 2N KHCO3 and 0.5 N NaAsO2 is an example of an immobilized liquid which would not be stable due to water evaporation. As a point of reference, flue gas from a typical coal-fired power plant is very sensitive to a few (2 to 3) inches of water back-pressure, and 0.5 psi is equivalent of about 16 inches of water. Values for polymeric membranes of 60 psi to achieve only a 3.1 barrier would be totally unacceptable for high gas flow applications such as power plant flue gas where gas compression would be totally impractical due the large gas volume. Instead, the design of a membrane system for coal-fired power plants must operate with minimal back pressure (for example, 2 to 3 inches of water).
The natural gas industry primarily uses solvent extraction processes to achieve gas separation. Referring to FIG. 5, the amine scrubber system 500 may pass all of the flue gas 410 through the scrubber 510 where the scrubber liquor 530 may absorb the CO2. The absorption process is a neutralization reaction, which is exothermic and requires a cooler 520 to keep the temperature low for efficient capture. The scrubber liquor 530 may pass to the recycle unit where heat 540 and/or reduced pressure are used to remove the CO2 and return it to the scrubber 510. There is some degradation of the ethanol amine that must be disposed of as waste 550.
For the reasons stated above, and for other reasons stated below which will become apparent to those skilled in the art upon reading and understanding the present specification, there is a need in the art for gas separation alternatives, and particularly as applied to carbon dioxide removal.
This background information is provided to reveal information believed by the applicant to be of possible relevance to the present invention. No admission is necessarily intended, nor should be construed, that any of the preceding information constitutes prior art against the present invention.